Cult -Cold Climate planting (planting depth)

Hi all,

I have some quick recent observations.

I planted about 200 TB and IB Iris about a month ago, supposedly all with
the rhizomes just under the surface, but as usual, dirt shifts and settles,
the dog steps on things, the wind blows, and (unbelievable) it has even
poured rain a couple times.  So, some of the rhizomes have their backs
uncovered, and some are buried deeper than they should be.  While this is a
mix of cultivars and not a very scientific test, I see the ones that are
buried the deepest (about 1 to 2 cm) growing by far the fastest and the
best.  Some of the exposed ones have lost all their foliage and are not
growing at all yet.

However, that is this year; it may not be the same next year.  It's been
considerably cooler than usual this year, and has been over 90 F only a
relatively few times since July (today and yesterday it just barely broke
90 F).  Also, it is late enough in the year that even when the days do top
90, the nights are a bit cooler, and the heat of the day doesn't last quite
so long.
A couple years ago my impression was that the buried ones were rotting
frequently and badly (mostly in August, some in September), but it was much
hotter.  The ones that were showing scalded a bit, but then were the ones
that did best later.

Last year I didn't loose many, but I was watching them more closely and
treating as soon as I found the first trace of rot.  Even so, a lot started
to rot, and most of those were covered with dirt.  Again the exposed ones
mostly lived, but started growing later and less vigorously.

There is also a very strong correlation between ancestry and rot here.  If
a plant has I. variegata ancestry (especially a large dose) it is more
likely to rot in the heat.

I never have problems in the winter with rot, heaving, or anything cold
related (except the occasional frozen flower on a confused plant or from a
late frost in early April), but then I am at the margin of USDA Zone 7 and
8, so we only get a few "severe" nights each winter (the ones below 15F,
occasionally briefly into single digit, rarely below 0F).

No rot at all on new plantings this year (fingers are crossed), but has a
few clumps seriously infected during hot weather in late June.   Seems that
approximately 90 F is the magic temperature.  If it stays cooler than that,
no problems.  Also, potted plants in afternoon shade seem to grow well with
no rot, even through the heat (but I have no trees to plant them under in
the yard).

Dave

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